Capacitance sensing systems can sense electrical signals generated on electrodes that reflect changes in capacitance. Such changes in capacitance can indicate a touch event (i.e., the proximity of an object to particular electrodes). Capacitive sense elements may be used to replace mechanical buttons, knobs and other similar mechanical user interface controls. The use of a capacitive sense element allows for the elimination of complicated mechanical switches and buttons, providing reliable operation under harsh conditions. In addition, capacitive sense elements are widely used in modern customer applications, providing new user interface options in existing products. Capacitive sense elements can range from a single button to a large number arranged in the form of a capacitive sense array for a touch-sensing surface.
Transparent touch screens that utilize capacitive sense arrays are ubiquitous in today's industrial and consumer markets. They can be found on cellular phones, GPS devices, set-top boxes, cameras, computer screens, MP3 players, digital tablets, and the like. The capacitive sense arrays work by measuring the capacitance of a capacitive sense element, and looking for a delta in capacitance indicating a touch or presence of a conductive object. When a conductive object (e.g., a finger, hand, or other object) comes into contact or close proximity with a capacitive sense element, the capacitance changes and the conductive object is detected. The capacitance changes of the capacitive touch sense elements can be measured by an electrical circuit. The electrical circuit converts the measured capacitances of the capacitive sense elements into digital values.
There are two typical types of capacitance: 1) mutual capacitance where the capacitance-sensing circuit has access to both electrodes of the capacitor; 2) self-capacitance where the capacitance-sensing circuit has only access to one electrode of the capacitor where the second electrode is tied to a DC voltage level. A touch panel has a distributed load of capacitance of both types (1) and (2) and Cypress' touch solutions sense both capacitances either uniquely or in hybrid form with their various sense modes.
An in-cell liquid crystal display (LCD) panel is a LCD panel that also includes the touchscreen functionality by positioning at least one of the touch layers (typically the transmit (TX) layer) under the color filter glass. Furthermore typically the TX layer is shared with the common electrode (VCOM reference layer) of the display. A touch screen controller (TSC) may be a capacitive touch screen controller that is used to measure the capacitance on an array of electrodes, such as an array including multiple transmit (TX) electrodes and multiple receive (RX) electrodes. A display driver integrated circuit (DDI) is typically the integrated circuit (IC) positioned on the substrate glass of the LCD which drives the timing and video signals to the LCD. While the above description is for in-cell LCD, similar stackups exist for other display types, such as active-matrix organic light-emitting diode (AMOLED).
In-cell panels typically use shared layers between touch and display functions to minimize the module thickness and cost. In practice, LCD vendors are implementing LCD VCOM and Touch TX on the same layer. This may have two repercussions. 1) Since VCOM is only driven during the active video part, the consequence is that the capacitance touch scanning can only occur during quiet video times, i.e. video blanking times. Therefore, a timing synchronization is needed between a TSC integrated circuit and a DDI. 2) The DDI now needs to drive not only the VCOM signal, but also the TX signal. Therefore, the TSC needs to inform the DDI of a TX pattern sequence for the TX signal to send out on the layer.